22nm Silicon-On-Insulator (SOI) complementary metal-oxide semiconductor (CMOS) technology has a number of performance boosters, such as third generation embedded DRAM, embedded stressor technology and 15 levels of copper interconnect.^[1] In addition to geometric scaling, strain engineering in CMOS transistors has provided another enabler for device performance improvement. A compressive strain in PMOS channel can increase the hole mobility, and a tensile strain in NMOS channel can increase the electron mobility. In PMOS of 22nm SOI technology, epitaxial SiGe source/drain (S/D) is used to introduce compressive strain in the channel, and SiGe layer in the channel is used to control threshold voltage of device. Hence obtaining the nanoscale chemical composition and strain information of them is vital during the semiconductor development.

    In this work, energy-dispersive X-ray spectroscopy (EDX) in STEM is used to determine the Ge atomic concentration (%Ge) based on the Cliff-Lorimer ratio method.^[2] The Cliff-Lorimer factor is calibrated by measuring a standard Si_0.664Ge_0.336 blanket sample with a relative error smaller than 1%. An improved high angle annular dark field (HAADF) STEM image is obtained by STEM with Drift Corrected Frame Integration (DCFI). DCFI technique integrates successive STEM images via calculating and correcting the drift from cross correlation. The produced STEM image has minimal drift and a high signal-to-noise ratio. It is analyzed by Geometrical Phase Analysis (GPA) to extract strain information.^[3]      

    Figure 1(a) shows a typical PMOS transistor with <110> in-plane direction and <001> out-of-plane direction. SiGe channel and dual-layer embedded SiGe source/drain (S/D) can be clearly observed in HAADF-STEM mode. Since HAADF image intensity is proportional to atomic number Z^1.7,^[4] SiGe layers with different %Ge are clearly seen, and can be further related to %Ge measured from EDX. In Figure 1(b), EDX map shows that channel SiGe has around 22% Ge. In S/D the buffer layer has 20-21% Ge, and the main layer has 25 – 30% Ge. HAADF micrograph and the corresponding deformation maps are shown in Figure 2. A compressive strain of 0.3% in the <110> direction is observed in the channel Si. There is no deformation between channel SiGe and channel Si in the <110> direction, suggesting that channel SiGe is compressed in this direction to match underneath Si lattice. This agrees with the finding that a deformation as high as 1.9% in channel SiGe is observed in the <001> direction. In addition the channel Si shows a tensile strain of 0.3% in the <001> direction. Results show that a combination of STEM-based techniques, including HAADF-STEM, STEM-GPA and STEM-EDX, can reveal the nanoscale chemical composition and strain distribution of a transistor. These information are used to monitor and control the process.

References

[1] S. Narasimha et al., IEDM (2012) p. 331.

[2] W. Weng et al., Microscopy and Microanalysis 21(S3) (2015) p. 1087.

[3] J.-L. Rouvière et al., Ultramicroscopy 106 (2006) p. 1.

[4] O.L. Krivanek et al., Nature 464 (2010) p. 571.

[5] Acknowledgements: The authors thank Joshua Bell (GlobalFoundries) for providing samples, and John Miller (GlobalFoundries) for preparing TEM lamellae. The authors also thank John Bruley (IBM), Yun-Yu Wang (GlobalFoundries), Frieder Baumann (GlobalFoundries) and Michael Gribelyuk (GlobalFoundries) for valuable discussions.

Figures:

Figure 1. (a) HAADF-STEM micrograph and (b) EDX map of a typical 22nm PMOS transistor showing SiGe source/drain, channel SiGe and a gate. It has <110> in-plane direction and <001> out-of-plane direction.

Figure 2. (a) Atomic resolution HAADF-STEM micrograph with <001> as Y-axis and <110> as X-axis, (b) <001> deformation map (εyy), (c) <110> deformation map (εxx) and (d) shear strain map (εxy) of a 22nm PMOS transistor.

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